Twenty-five years ago next week, humanity sent its first and only deliberate radio message to extraterrestrials. Nobody has called back yet, but that's OK -- we weren't really expecting an answer.

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The message was sent during the dedication of a major upgrade to the Arecibo radio telescope in Puerto Rico on the afternoon of Nov. 16, 1974, and contained some very basic information about the human race. It included representations of the fundamental chemicals of life, the formula for DNA, a crude diagram of our solar system and simple pictures of a human being and the Arecibo telescope.

"It was strictly a symbolic event, to show that we could do it," explains Donald Campbell, Cornell University professor of astronomy, who was a research associate at the Arecibo Observatory at the time. Arecibo Observatory is operated by the National Astronomy and Ionosphere Center, managed by Cornell University for the National Science Foundation.

The real purpose of the message was to call attention to the tremendous power of the radar transmitter newly installed at Arecibo and the ability of the telescope's 1,000-foot diameter dish antenna to project a powerful signal into space. But many of those present took the event seriously, according to Harold Craft, Cornell's vice president for services and facilities, who was then director of the Arecibo Observatory. "We translated the radio-frequency message into a warbling audio tone that was broadcast over speakers at the ceremony. When it started, much of the audience spontaneously got up and walked out of the tent and gazed up at the telescope."

While the audience that had gathered beside the huge Arecibo dish was impressed by the idea of sending messages to space, others were critical. Some actually suggested that sending such a message was dangerous, because it might attract the attention of hostile aliens.

They probably needn't have worried. The chance that the message might actually be detected by some extraterrestrial intelligence is extremely small. It was sent only once, over a period of about three minutes, on a narrow beam directed toward a group of about 300,000 stars called the Great Cluster in Hercules, Messier 13. The globular cluster is 25,000 light-years away in our galaxy, the Milky Way. So far, moving at the speed of light, the message has traveled only one thousandth of the distance, or about 147 trillion miles. There are stars closer to our solar system than that, but none of them is in the path of the message.

Ironically, the globular cluster at which the signal was aimed won't be there when the message arrives. It will have moved well out of the way in the normal rotation of the galaxy. But "anyone" in the target area when the signal arrives could detect it with a radio telescope of similar size, and it would appear at 10 million times the intensity of the normal radio signals from our sun. From there, the message will continue on its course through outer space, ultimately, millions of years hence, reaching distant galaxies.

Since the transmitter was installed in 1974, Arecibo radar has been used for extensive explorations of the solar system, including detailed mapping of the surfaces of the moon and Venus. The radar was upgraded to even higher power in 1997. No other formal messages have been sent, but many of the radar signals have continued on out of our solar system and if detected would clearly be seen as created by intelligent beings, Campbell says. In addition, a message, engraved on copper plate, accompanied the Pioneer 10 spacecraft launched in March 1972 and now is about 7 billion miles from Earth.

Meanwhile, researchers constantly use the huge dish antenna to listen for signals from alien intelligence. One project, known as Phoenix, aims the telescope at specific stars; another, called Serendip, collects data on certain likely frequencies during all the telescope's other operations, and distributes the data to thousands of volunteers to process on personal computers. Project Phoenix is directed by the non-profit SETI Institute, based in Mountain View, Calif. Serendip is a project of the University of California at Berkeley.

The 1974 message was transmitted on a frequency of 2380 MHz and consisted of 1,679 binary bits representing ones and zeros, sent by shifting the frequency of the signal up and down over a range of about 10 Hz, a method similar to that used by computer modems to send binary code over a telephone line. If the ones are translated into graphics characters and the zeros into spaces, the message forms a symbolic picture 23 characters wide by 73 long.

The content of the message was developed by Frank Drake, then professor of astronomy at Cornell and now a professor in the Division of Natural Sciences at the University of California at Santa Cruz and president of the SETI Institute; Richard Isaacman, then a Cornell graduate student and now working at Integral Systems Inc. in Lanham, Md.; Linda May, another graduate student now professor of physical sciences at Wheelock College in Massachussetts, and James C.G. Walker, then a member of the Arecibo staff and now professor of physical sciences at the University of Michigan at Ann Arbor. Others, especially the late Carl Sagan, who eventually became the David Duncan Professor of Astronomy and Space Sciences at Cornell, contributed to the project.

Editor's Note:

After issuing this news release we were informed that a new message to extraterrestrials was sent in May of this year, using a Russian radio telescope. So, the Arecibo message is no longer the only such attempt. A BBC story on the new message is on the web  here. This link may expire. A private web site describing the project is at http://pages.infinit.net/lachapel/seti/introduction.html. An organization called Encounter2001 has announced plans for more messages.